Field of the Invention
The invention concerns a method for hot-dip coating of a steel flat product with a metallic protective coating, wherein the steel from which the steel flat product is made contains at least 2.0 wt. % Ni and at least 5.0 wt. % Cr.
The term “steel flat products” used here refers to strips or sheet steels, and the plates and blanks produced therefrom.
Description of Related Art
In the same way for example as Mn or N, Ni stabilises the austenitic structure state in steels at lower temperatures. This effect can be deliberately used to improve the mechanical material properties of the steel. Multiphase steels with residual austenite proportions have a particularly good combination of strength and ductility. Fully austenitic steel qualities with Ni content >8 wt. % furthermore have no brittle-ductile transition, which allows low-temperature applications. In comparison with high Mn-alloy steel qualities, Ni-alloy full austenites are furthermore substantially less susceptible to environmental loads. In particular, Ni steels with an additional Cr-alloy proportion are characterised by particularly good chemical resistance and high corrosion resistance. The presence of Mo in the respective steel further supports this passivation. Further elements such as Al, Mn, Ti and Si may be added, depending on the desired material strength or ductility of the respective steel alloy.
Because of its particular material properties, there is great potential for use of Ni-alloy steel flat products in the region of high- and low-temperature applications. These include amongst others vehicle construction, in particular the structural components in the suspension area, chemical equipment construction, plant and machinery construction. Furthermore for example decorative elements for housebuilding or similar can be produced using Ni-alloy steel.
Despite its significant resistance to environmental influences, when using steel flat products produced from nickel-alloy steels of the type described here for particularly stressed components or parts, it may be technically necessary or economic to apply an additional protective coating. This not only optimises the resistance, in particular the corrosion resistance, but also improves the forming suitability or aesthetic appearance of the respective steel flat product.
Continuous strip coating constitutes a generally established method of applying such a metallic protective coating to a steel flat product. However the chemical passivity of external oxides of the alloy elements, which adhere to the respective surface of the product to be coated, causes a deterioration in the coating result. Critical alloy constituents in this respect are for example Cr, Al, Mn, Si and other oxide-forming elements. The oxides formed from these alloy elements on the surface of the steel flat product to be coated cause wetting and adhesion defects. To avoid these defects, particular requirements must be fulfilled by the method and plant available for continuous strip coating.
AT 392 089 B describes a method of electrolytic galvanisation of stainless steels on one and both sides in the continuous strip process. This method is however comparatively costly and is therefore not widely used in practice.
A more economic alternative to electrolytic coating is continuous hot-dip coating of strip steels. In this method, after undergoing recrystallisation annealing in a passage furnace, a steel strip is briefly immersed in a metallic melt bath which is typically based on zinc, aluminium or their alloys.
The hot-dip coating of alloy steels requires particular care since during the annealing phase, in these steels, oxygen-affine alloy constituents can oxidise selectively on the steel surface. If the selective oxidisation takes place externally, wetting defects and adhesion faults must be expected.
To avoid these problems, steel flat products which are to be given a protective metal coating by hot-dip coating must normally have their alloy content limited to specific maximum values. This includes the Ni content of the respective steel substrate. The Ni content of steel flat products to be given a metallic protective coating by hot-dip coating is usually limited in practice to less than 2.0 wt. %, in particular up to 1.0 wt. %. Examples of this are given in EP 2 009 128 A1, EP 1 612 288 A1 and EP 2 177 641 A1.
A different special case is the hot-dip coating of nickel-alloy steels which, as well as Ni, also contain Cr in contents of 5-30 wt. %. For such Cr-alloy steel qualities, attempts have been made for example to suppress the external oxide formation by heat treatment with strongly reductive annealing parameters (so-called bright annealing). Methods based on this concept are described in U.S. Pat. Nos. 4,675,214, 5,066,549, 4,883,723, 5,023,113 and EP 0 467 749 B1.
As an alternative to these known methods, JP 3 111 546 A and JP 5 311 380 A each propose forming a targeted FeO layer (pre-oxidisation) during heating, and reducing this layer to metallic iron (Fe) during a subsequent holding phase.
Furthermore in the method known from U.S. Pat. No. 5,591,531, an Fe-rich edge zone of the steel flat product to be coated is preconditioned by means of hood annealing in an off-line working step. Then the steel flat product is introduced into a hot-dip coating plant and coated.
Another possibility cited in EP 2 184 376 A1 is an off-line pre-coating in which a thin Fe layer is applied to the surface of the steel strip (“Fe flash”).
The methods outlined above for hot-dip coating of Cr-alloy steels require the hot-dip coating to be carried out as a thermal aluminisation, but do not usually cover the application of a zinc-based protective coating. The practical use of the methods explained above for hot-dip coating of Ni-/Cr-alloy steels in practical operation is hindered by the problem that they can only be carried out at great expense on a conventionally designed hot-dip coating plant which is designed for conventional alloy steels. In addition there are high consumption and maintenance costs, which are caused by the high H2 consumption associated with the use of the known methods and the high annealing temperatures required for these methods. The high consumption and operating costs do not harmonise with today's demand for economic and ecological compatibility of such methods. In particular, operational trials show that pre-oxidisation in the manner of the procedures described in JP 3-111546 A and JP 5-311380A is difficult to manage in practice, and offers low process reliability. Both in the case that too thick an oxide layer is formed during oxidisation, and if the layer thickness is too low, direct wetting defects can occur which cause poor adhesion of the hot-dip coating to the respective steel substrate.
In principle, the known proposals outlined above for hot-dip coating of Cr-alloy steels relate only to ferritic special steels. The Ni content of the steel alloys concerned—where there is any Ni content at all—is consequently limited to a comparatively low upper limit of <3.0 wt. %. It is known from the prior art that the presence of Ni can be extremely advantageous in a steel flat product to be coated with hot-dip coating. A precoating of the steel strip surface with a thin Ni layer applied before insertion of the strip steel in the hot-dip coating plant (“Ni flash”) effectively suppresses the selective oxidisation of oxygen-affine alloy elements of the steel flat product and persistently improves the coating result. The use of an Ni flash for hot-dip coating is recommended e.g. in JP 61-147865 A or JP 60-262950 A. Since the application of an Ni flash however requires an additional working step, which entails substantial additional costs, this procedure has not become common in practice.
As well as the separate application of an Ni flash, the Ni content of the steel alloy itself can be used to improve the quality of a metallic hot-dip coating. According to U.S. Pat. No. 7,736,449 B2, an Ni content of up to 2.0 wt. % Ni is added as an alloy to the steel of the steel flat products to be coated in order to inhibit the occurrence of Mn, Al and Si oxides on the surface of the steel flat product. The respective Ni proportion to be added is defined by various complex, formulaic correlations both with the contents of the other alloy elements of the steel flat product and with the annealing parameters which must be observed during recrystallisation. A similar approach is followed in WO 00/50658 A1 for thermal galvanisation of the steel flat product with 0.2-5.0 wt. % Ni. In this method too, an edge layer is produced on the surface of the strip steel, in which the Ni content is set as a function of the respective Al and Si content. The dependencies to be taken into account in process management mean that this method cannot be implemented on a large industrial scale with the necessary operational reliability and reproducibility. In this context, it was an object of the invention to specify a method which allows the hot-dip coating of Ni-alloy steel flat products in a cost- and resource-effective manner, and which can be used process-reliably in industrial practice.